Bolometer



May 9, 1951 E. M. woRMsER BOLOMETER Filed Sept. 29, 1954 @Qn/8 ATTORNEY l INVENTOR .Erz/c M. Wormser il nited States Patent Oce 2,983,888 Patented May 9, 196i BOLOMETER Eric M. Wormser, Stamford, Conn., assigner, by mesne assignments, to Barnes Engineering Company, a corporation of Delaware Filed Sept. 29, 1954, Ser. No. 459,076

14 Claims. (Cl. 338-18) This invention relates to an improved construction for radiation sensitive devices. More specically, it relates to improvements in thermistor bolometers in which the Window of the bolometer housing serves also as a thermal sink as Well as a support for the sensitive element.

Thermistor bolometers are useful for measuring infrared radiation, and thus can be used for all types of temperature measurements. Previously the best thermistor bolometers consisted of a thin iiake of resistance material with a high negative temperature coeicient cemented to a backing block of heat conducting material. It was the general practice to mount the flake backing block assembly in a metallic housing having a window of infrared transparent material. Also the hacking block, flake support and the window were sometimes combined into a single unit by mounting the ake on the inner surface of the Window in which case ythe outer surface of the window could be shaped to act as a lens, thereby focusing the incident radiation on the relatively small ake or flakes mounted near the central portion thereof. Infra-red radiation impinging on the flake through this window caused it to increase in temperature, thus changing its resistance. This resistance change was detected by applying a direct biasing voltage to the flake and measuring the current variation caused by the change in resistance. The window served as a thermal sink to carry heat away from the ake.

For most applications, thermistor bolometers should have a fast response rate to detect rapid changes in temperature or pulses of infra-red energy. This response rate is determined, in part, by the heat conducting charracteristics of the material used as a thermal sink and I have discovered certain materials, which, when used for this purpose have much greater thermal conductivity than those previously used. When such materials are used the thickness of the cement layer becomes a factor of vital importance as will presently be described in detail.

The thermal conductivity of the thermal sink also determines in part, the steady state heat dissipation from the flake. The steady state heat `dissipation in turn determines the maximum bias voltage which may be applied to the flake. Because the signal and the signal-to-noise ratio of these devices is directly proportional to such bias voltage, it is advantageous to have a thermal sink with the highest thermal conductivity possible.

Previously where the thermal sink, flake support and window Were combined as a single unit, the materials used therefor had poor thermal conductivity. One such material is selenium, which, although transparent to at least a portion of the infra-red spectrum, has very poor thermal conductivity. Accordingly, its use resulted in thermistor bolometers with slow response rates, compared with those of bolometers having thermal sinks of material which is not transparent to infra-red radiation. Such thermistor bolometers have not been used where fast response rates are desired.

Accordingly, it is an object of this invention to provide a thermistor bolometer in which the housing window, ake support and thermal sink are combined in a single element having a greatly improved response rate. Another object of this invention is to provide a device of the-above character capable of operating with higher bias voltages to give improved signal-to-noise ratios. A further object of this invention is to provide a device of the labove character by utilization of improved materials for the thermal sink. Another object is to provide a device of the above character in which the resistance element has improved uniformity of response over the exposed area. Another object of this invention is to provide Window materials for use as infra-red lenses, which have higher indices of refraction than those heretofore used. A further object of this invention is to provide materials for use as combined windows and thermal sinks having a coeicient of expansion similar to that of the thermistor material, thus permitting the use of the window-ake assembly over wide ranges of temperatures. A further object of this invention is to provide a simplified heat-sensitive image tube of economical construction having a fast response rate. A still further object of this invention is to provide, in an image tube, an infra-red transparent face plate which will also serve as a support for infra-red sensitive elements. A still further object of this invention is to provide an infra-red image tube in which the sensitive elements may be scanned directly by the scanning means. Other objects will in part be obvious and in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the constructions hereinafter set forth and the scope of the invention will be indicated in the claims. For a fuller understanding of the nature and objects of this invention reference should be had to the following detailed description taken in connection with the accompanying drawing in which:

Figure 1 is a front elevation of my improved thermistor bolometer having the features of this invention incorporated therein,

Figure 2 is a vertical sectional view of my thermistor bolometer taken roughly -along the line 2 2 of Figure 1, except for the lead details which are shown completely in elevation for purposes of greater clarity.

Figure 3 is an enlarged fragmentary vertical sectional view of the portion of my thermistor bolometer in the vicinity of the flake showing with greater clarity how the flake is attached to the window,

Figure 4 is a fragmentary view partially in section of one end of an infra-red image tube having another embodiment of my invention incorporated therein,

Figure 5 is an enlargement of the section between the lines 5 5 of Figure 4 showing the mounting of strips of thermistor material in the tube,

Figure 6 is a fragmentary rear elevation of the tube face showing the electrodes connected to the strips of thermistor material, and

Figure 7 is a fragmentary vertical sectional view of a modification of the embodiment shown in Figure 2.

Similar reference characters refer to similar parts throughout the several views of the drawings.

Generally speaking, I have discovered that two factors are responsible for the poor speed of response of prior thermistor bolometers combining the housing window, ake support and thermal sink. These were the poor thermal conductivity of the infra-red transparent windows and the lack of flatness of the thermistor flakes which resulted in unduly thick cement layers.

These non-at thermistor flakes are the result of the sintering process which they undergo during manufacture. Although the flakes are flat when entering the sintering furnace, they encounter high temperatures during the sintering process which warp them and cause their edges to curl. For example, in a standard Hake of 10 microns thickness this curvature would generally range between 15 and 200 microns measured from the lower surface of the body of the flake near the central portion thereof to the lower surface of the curled edge. When a Hake of such curvature is cemented to the transparent window with the thinnest possible layer at the central portion thereof the upward curl of the edges requires a much thicker cement layer adjacent thereto. If the Hakes are placed on the window with the edges curled downward, the thickest cement layer will correspondingly be at the central portion. This results in cement layers whose average thicknesses are approximately'two or three times the minimum possible thickness. These fairly thick cement layers did not appreciably affect bolometer performance when materials having a relatively poor thermal conductivity, such as selenium, were used for the combined window and thermal sink. However, when other materials having higher thermal conductivity are used a cement layer 1/200 as thick-as the window sometimes has the same thermal resistance. Under such circumstances the reduction of the thickness of the cement layer is of major importance.

I propose to use Hakes which are Hat to within microns or less thereby appreciably reducing the thickness of the cement layer. Preferably these Hakes are cemented to a Hat polished surface of an infra-red transparent window and this results in a uniform thin cement layer. In practice it is preferable but not essential that the polished surface of the window be optically Hat. Since the Hakes are Hat to within 5 microns andvsince 5 microns is of the order of magnitude of one wave length in the infra-red spectrum, the Hakes are optically Hat to infra-red energy. Therefore, Hakes which are Hat within 5 microns or less will hereinafter be referred to as optically Hat in the sense of certain features of this invention.

lFat Hakes of this type can be obtainedby processing curled Hakes. Thus a plurality of curled -Hakes may be placed between ground and polished optically Hat high temperature glass plates and heated to about 1000? C. under light pressure for a few minutes; then they are cooled slowly. Most of the Hakes so processed will be optically Hat and hence suitable for use in my improved thermistor bolometers. With such Hakes, it is possible to obtain cement layers of the order of 3 to 10 microns in average thickness and layers which are uniform in thickness to within 5 microns or less.

As previously mentioned, the other major factor in- Huencing the speed of response of this type of thermistor bolometer is the thermal conductivity of the window. Besides being infra-red transparent and a ygood conductor of heat, it should be a good electrical insulator. It should also have a coefficient of thermal expansion similar to the thermistor material. -All of these properties are not usually found in a single material, but I have found certain materials combining them in varying degrees thereby providing greatly improved rates of response for such thermistor bolometers. With these thin cement layers such as I am able to achieve, conventional backing materials such as selenium, show some slight improvement in rate of response. More important, however, materials with much higher thermal conductivities and having the other requisite properties exhibit correspindingly faster rates of response.

Both the use of optically flat Hakes and materials of greater thermal conductivity serve to increase the speed with which heat is carried away from the Hake during operation of the bolometer. One of the disadvantages normally incident to increasing this speed is a loss in responsivity. More particularly the temperature change of the Hake is not as great since heat is carried away faster and the resistance changes are therefore not as great resulting in a lower signal output for the same biasing voltage. However, as previously mentioned, the permissible biasing voltage can be increased as the heat dissipating characteristics of the device are improved. Thus although resistance variation decreases, the signal may be increased because a higher bias voltage can be used in my construction with its superior dissipation characteristics. This usually results in some increase in signal-to-noise ratio and much faster response rates.

Referring to the drawings in detail, and particularly to Figures l and 2, the construction here shown may be used to house and support my improved thermally sensitive device, it being understood that other structures could be utilized for this purpose. As shown herein a housing for a thermistor bolometer is generally indicated at 10, comprising a Hat base 12, secured to a cylinder 14 to form a housing for the other parts of the device. Cylinder 14 has annular bores `16 and 1S adjacent its ends, base 12 resting in bore 18, and being held in place by solder seals or fillets 20 and 22. A window 24 made 0f infra-red transparent material and having a high thermal conductivity but a low electrical conductivity is cemented in the bore 16. As will be presently described in greater detail a thermally sensitive Hake 26 is cemented to the under side of the window 24. The ends of the Hake 26 have gold coats 56 and 58 (Figure 3) and leads 28 and 30 are connected thereto and to larger leads 32 and 34 which in turn are connected to pins 36 and 3S. These pins are supported in holes 40 and 42 in the base 12 by glass seals 44 and 46. These seals are connected to metal base 12 by solder seals 48 and 50 and pins 36 and 38 are preferably shaped and located to plug into a standard tube -socket or the like. Base 12 is preferably copper or steel alloy and glass seals 44 and 46 have substantially the same temperature coeiiicient of expansion as the base to avoid heat damage. Accordingly, the device may be connected into a circuit so that biasing voltage may be impressed across the Hake 26 and signals from the flake ampliHed. Structural details of the thermally sensitive portion of the construction may be more readily comprehended from an examination of Figure 3 in which certain of the dimensions are exaggerated for purposes of greater clarity. Flake 26 is attached to the window 24 by a cement layer 52 of minimum thickness. The rear surface of Hake 26 may be coated with a black infra-red absorbing layer 54 to absorb any infra-red energy not absorbed by the Hake. Infra-red energy impinges on the Hake through window 24 and is absorbed by cement layer 52 and Hake 26, the excess, if any, being absorbed by the black layer 54. From such infra-red absorption the flake increases in temperature causing a change in its resistance which may be detected from the change in current Howing through the Hake.

The desirable operating characteristics of my thermistor bolometer are due in part to the precise physical characteristics of certain of the parts now to be described. Thus, Hake 26 is a resistor having a high negative temperature coeicient and the ability to change in resistance value when infra-red rays impinge thereon. Preferably a mixture of oxides of manganese, nickel, and perhaps cobalt is used in making such Hakes. Such mixtures are not conveniently expressed by weight since the oxidation of the metals is not precisely known. It is preferably defined by the number of atoms of the particular metal present per 100 atoms of the mixture. On this basis, the preferred resistance materials are manganese to 20 nickel, and 52 manganese to 16 nickels and 32 cobalt.

As previously mentioned, the Hake 26 should be optically Ha which term asV used herein means that the Hake should pass between two plane parallel surfaces Vspaced apart no more than 5 microns greater than the Hake thickness. For example, the standard 10 micron Hake used in thermistor construction, to be termed optically Hat, should pass between two plane parallel Vsur- 5 faces" 15 microns apart. Flakes with greater departures `from such ilatnessV are termed non-ilat or curled The akes are preferably generally rectangular in shape; typical iakes vary from l to 0.05 mm. in length and from l0 to 0.05 mm. in width.

Returning now to Figure 3 of the drawings, cement layer 52 is of minimum thickness, i.e. as thin as possible while still performing its adhesive function. In practice I have found that the average thickness of this layer should not be greater than 10 microns and preferably should be about 5 microns. Layers in such range of thickness are hereinafter termed thin The cement layer should also be as uniform las the variation in flake atness will permit. 'I'he ake and the adjacent surface of the windowcould be curved or non-flat provided the facing surfaces correspond so that the space therebetween is uniform to Vwithin microns to assure a thin cement layer, for the cement layer therebetween will then be thin in the sense of this invention. Not only must the cement establish a strong bond between the akes and the window butvit should olier minimum resistance to heat ow. It should be an insulator to avoid short circuiting theake, but it is not desirable to depend upon it for such vinsulation'. 1 Plastic resins are preferred as a general class of materials to be used for this cement because they provide a strong but flexible bond between the flake and the backing block over wide temperature ranges. Their thermal conductivity is of the order of 5 X10'-4 cal/sec. C. cm.2 per cm. of length as compared with selenium for example which has a thermal conductivity of somewhat 4'less than 3 X10-4 cal./ sec. C. cm? per cm. of length.

Accordingly a cement layer slightly thicker than a selenium window oers about the same resist-ance to heat flow; of course in the actual bolometer, the cement layer is much thinner than the window. Thus when selenium is used as a combined window and thermal sink, as previously mentioned, the thickness of the cement layer has no important effect on bolometer rate of response.

However, when the thermal sink has a higher thermal conductivity, the thickness of the cement layer is of great importance since, as stated before, a cement layer 1/200 as thick as some of the improved materials used for windows may have the same resistance to heat ilow as the window. Thus, in order to take full advantage of the higher thermal conductivities available with these irnproved materials it is highly desirable to minimize the thickness of the adhesive layer.

The general class of materials suitable for use as a window 24 should have a high thermal conductivity, preferably comparable to that of metals, but low electrical conductivity, preferably comparable to that of electrical insulators. In addition, they must be transparent to infrared radiation. The ideal would be high thermal conductivity, complete infra-red transparency and excellent insulating characteristics, but some compromise must be made in a material combining such characteristics for they are usually incompatable in the same substance. The window and the akeshould have approximately the same thermal coeicient of expansion so that they will similarly expand and contract with variations in temperature. This permits use of the device over wide ranges of ambient temperatures. Most of the improved materials which are discussed below have coeflicients of thermal expansion similar to that of the thermistor material. Selenium which was previously used, has a coeiiicient of thermal expansion over four times Ithat of the thermistor material; as a consequence the permissible variation in ambient temperature was limited.

I have found that magnesium oxide meets these rigid requirements for a window in my improved device in an excellent manner; it is an electrical insulating material, is transparent to infra-red energy out to a wavelength of -8 microns, has a thermal conductivity of the order of 0.1 cal/sec. C. cm.2 per cm. of length and its thermal coeicient of expansion is about the same as that of the thermistor material previously mentioned. This thermal conductivity is about 300 times greater than selenium. By the use of such material I have produced thermistor bolometers of the above construction with response rates characterized by time constants of the order of one-half to two milliseconds compared to time constants of nine to twelve milliseconds for selenium. Another excellent material for use in the window is germanium. It has a thermal conductivity of approximately 0.14 cal./sec. C. cm.2 per cm. of length, being slightly greater than magnesium oxide. In its pure form it is transparent to the infra-red spectrum from 1.8 to beyond l5 microns. Impurities in the germanium decrease this transparency. A thin electrically insulating infra-red transparent layer is preferably interposed between the flake and the germanium window to provide the necessary insulation, since this material is a semi-conductor. Germanium has an index of refraction to infra-red energy of 4 as compared to 2.4 for selenium. Since the optical gain of a lens varies with the square of the index of refraction, this means that germanium lens-shaped windows like that shown in Figure 7, will give optical gains almost three times as great as those provided by selenium windows. Germanium has a coeliicient of thermal expansion of 6.1 X l06 C. as compared to 8 to 9 l0*6 C. for the thermistor material so that the germanium window is useful over wide ranges of ambient temperatures.

Other infra-red transparent materials which might be used in this application are sapphire, silicon, crystalline sodium chloride and alloys of germanium and silicon. Silicon and alloys of germanium and silicon may require a thin insulating coating between the flake and the thermal sink similar to germanium.

Thus, I have provided an improved thermistor bolometer in which the backing block for the thermistor flake and the ake support and housing window are all combined in a single unit. Because of the improved material used for the window and the use of optically at flakes with thin cement layers between the window and the llakes, this device has much faster response rates than previous devices of this type.

A modified embodiment of the invention employing a lens-shaped window element 24a is shown in Figure 7. Curved lens surface Z5 here refracts incident rays 23, focusing them on flake 26.

Another embodiment of my invention utilizing iiat strips of thermistor material supported on an infra-red transparent face plate or window is illustrated in Figure 4 showing an infra-red image tube of conventional construction except for its face plate. This face plate 60 is formed from germanium, magnesium oxide or other infra-red transparent thermally conducting material. The face plate is attached by a graded glass seal 62 to the glass side 64 of the tube. A plurality of strips 66 of thermally sensitive resistance material, are cemented to the inside surface 60a of plate 60. Strips 66 are preferably shaped to conform to the optical curvature of surface 60a and are attached to this surface of face plate 60 by `a thin cement layer in the manner I have previously described in detail. Infra-red energy impinging on the face of the tube is transmitted through the face plate where it strikes the thermally sensitive strips. These strips change their resistance according to the change in temperature caused by this energy. In addition the strips are excited by an electric potential. In this iapplication electrodes on the interior surface of the strips 66a are scanned with an electron beam (not shown). The current in the electron beam depends upon the resistance of the thermistor element between adjacent electrodes. Thus as the beam scans the electrodes on the strips the current in the beam varies and such variations may be detected. Knowing the position of the beam and the current variations a picture of the infrared energy impinging upon the tube face is obtained and may be displayed or recorded by conventional means.

Prior devices of this sort used thermistor flakes or strips cemented to backing blocks as the sensitive elements. The backing blocks usually were cemented to the rear surfaces of the akes or strips, i.e. that surface exposed to the interior of the tube, thus blocking the beam so the thermally sensitive material could not be scanned directly from the rear. This resulted in complicated electrical circuits in order to obtain the desired panoramic heat picture. However, by utilizing an infrared transparent tube face as the thermal sink for the thermistor material, such expedients are unnecessary,

The manner in which the flakes or strips are attached to the transparent face plate 64b depends upon the scanning method used. One embodiment is shown in Figures and 6 where the strips of thermistor material 66 are attached to the face plate 60 by a thin layer 70 of cement, preferably similar to layer 52.

If germanium or silicon or other semi-conductors are used for plate 6G, the surface of the face plate 60 should be coated with an insulating layer 72 (Figure 5) which is not necessary if plate 60 is an insulator such as magnesium oxide.

Attached to each thermistor strip 66 are two sets of electrodes 68 and 69, preferably gold. As best seen in Figure 6, electrodes 68 are solid bars extending throughout the length of the strips 66 and are preferably fed from a common source of potential. Further, electrodes 69 on the opposite edges of the strips 66 from electrode 68 consist of a series of separate pieces of conducting material spaced along the edge of strip 66. The interior surface 66a of the strips 66 is preferably coated with a layer 74 of black infra-red absorbing material.

As seen in Figure 6, the electrodes 68 of adjacent strips 66 are mounted on adjoining edges as are the electrodes 69 so that they are substantially spaced from each other. In use electrodes 68 are preferably raised to a high positive potential, while electrodes 69 are usually at ground potential thus making for a very high potential gradient therebetween but the possibility of breakdown and consequent damage is avoided by this arrangement for adequate spacing as described.

In operation, infra-red energy impinging on face plate 60 is transmitted to the strips 66 effecting temperature changes and consequent resistance variation therein. Electrodes 68 impart positive potential `to one edge of strips 66 while an electron beam scans electrodes 69. As the beam scans each electrode 69, current in the beam varies according to the resistance between the electrodes as determined by strips 66. Such resistance, of course, depends upon the infra-red energy impinging on that particular portion of strip 66. Therefore current in the beam as it contacts any given electrode depends upon the infra-red energy impinging upon that particular portion of strip 66 associated with the electrode 69, which is thus a function of the incident infra-red energy impinging upon the plate at that point. 'I'he black layer 74 Von the interior surface 66a of the strip 66 absorbs any infra-red energy not absorbed by the thermistor strip and thereby enhances the absorption eiciency of the strip.

In another embodiment (not shown) strips 66 could be separated into individual ilakes with an individual electrode 69 on each flake and a common electrode 68 for all flakes in each row. Alternatively, the entire surface 60a could be coated with a layer of thermistor material in which case electrodes 68 and 69 are dispersed over the inner surface of the thermistor material to divide the layer into a plurality of individual thermally sensitive elements. Y

Where the entire surface 60a is coated electrodes 68 and 69 may be arranged to form a plurality of pairs of sensitive elements of thermistor material. Thus, electrodes 69 are centered between two electrodes 68. One of these electrodes 68 is then excited from a source of positive potential, and the other from a source of negative potential to form a plurality of paired sensitive eleas high speed switching or the like.

ments. One element of each of the pairs of sensitive elements thus formed may then be shielded to serve, as a compensating resistor for the exposed portion.

Y Whatever the larrangement of the thermistor material on the surface 60a, a layer of dielectric material may be placed overthe electrodes 69 to thereby form a plurality of capacitors in series with the thermistors.V The infrared energy impinging upon face 60 could then be chopped, i.e. periodically interrupted by a spoked rotating disc or the like. Alternatively, where paired elements as described above are used and their electrodes are covered with a layer of dielectric material, the optical image impinging on the face plate 60 could be oscillated by well known techniques between the negatively and positively biased thermistor elements. This would` serve to generate an alternating signal. Such chopped or oscillated infra-red energy would vary'the element resistance at the chopping or oscillating rate and cause a corresponding variation in charge of the condenser. As the dielectric material is scanned by an electron beam the variation in charge of the plurality of condensers can be ydetected and used to display or record`the heat energy interposition of the layer 74. The face plate is then excited from a source of electrical potential and serves as ta common electrode for all the resistor elements attached thereto. A plurality of electrodes are attached to the interior surface of the thermistor material. When these electrodes are scanned, the beam current variations correspond to the resistance of the thermistor material.

Other known methods of scanning the thermistor material not involving an electron beam may be used such The essential features of my improved infra-red image tube are the use of thin cement layers, to bond thermally sensitive material to the infra-red transparent face plate and the use of materials with high thermal conductivity for the face plate to provide a thermal sink for the sensitive material to make unnecessary the use of backing blocks as thermal sinks on the thermistor material. Y t

It will thus be seen that the objects set forth-above, among those made apparent from the preceding description, are efficiently attained. Since certain changes may be made in the above constructions withoutdeparting from the scope ofthe invention, it is intended that all matter contained in the above description, or shown in the accompanying drawings shall be interpreted `as illustrative and not in a limiting sense. It is also to be understood that the lfollowing claims are intended to cover all of the generic inventions herein described, and all the statements of the invention, which as a matter of language might be said to reside therebetween.

Having described my invention, whatI claim as new and desire to secure by Letters Patent is:

1. In a bolometer, the combination of an infra-red transparent windowV formed of material having high thermal conductivity and having a front surface for direct exposure toinfra-red radiation and a rear surface,

an optically flat thermistor ake capable of passing between two plane parallel surfaces spaced apart no more than five microns greater than the thickness of said flake, electrical contacts on said ake, and la thin adhesive layer not more than ten microns in thickness joining said ake to said rear surface of said window, whereby said window acts as a heat sink for said ake and changes in the infra-red energy passing Vthrough said window affect said flake to produce detectable resistance changes therein.

2. The combination defined in claim l lin which said window is magnesium oxide.

3. The combination defined in claim 1 in which said window is silicon together with a thin infra-red transparent electrically insulating layer interposed between said thermistor and said window.

4. The combination defined in claim 1 in which said Window is germanium together with a thin infra-red transparent electrically insulating layer interposed between said thermistor and said window.

5. The combination defined in claim 1 in which said window is crystalline sodium chloride.

6. The combination defined in claim 1 in which said Window is an alloy of germanium and silicon together with a thin infra-red transparent electrically insulating layer interposed between said thermistor and said window.

7. The combination defined in claim l in which said Window is synthetic sapphire.

8. The combination defined in claim 1 in which said thin adhesive layer is a plastic resin.

9. The combination defined in claim 1 in which said Window and said ake have similar coefiicients of thermal expansion.

10. The combination defined in claim 1 in which said thermistor liake is composed of oxides of manganese, nickel and cobalt.

11. In a bolometer, the combination of an infra-red transparent window formed of material having high thermal conductivity and having a front surface for di rect exposure to infra-red radiation and a rear surface, said front surface being lens-shaped to focus incident infra-red radiation, an optically flat thermistor fiake capable of passing between two plane parallel surfaces spaced apart no more than five microns greater than the ake thickness, and a thin adhesive layer not more than ten microns in thickness joining said ake to said rear surface near the focal point of said window, whereby said Window acts as a heat sink for said ake and changes in the infra-red energy passing through said window affects said ake to produce detectable changes.

12. The combination defined in claim 11 in which said lens-shaped window has a high refractive index.

13. The combination defined in claim 11 in which said 10 lens-shaped window is formed of material selected from the group consisting of germanium and silicon and alloys thereof.

14. The combination defined in claim 11 in which said lens-shaped window is formed of material selected from the group consisting of magnesium oxide, silicon, germanium, crystalline sodium chloride, alloys of germanium and silicon, and sapphire.

References Cited in the file of this patent UNITED STATES PATENTS 2,041,816 Carpenter et al. May 26, 1936 2,055,017 Praetorius et al Sept. 22, 1936 2,165,025 Baldwin July 5, 1939 2,213,175 Iams et al Aug. 27, 1940 2,238,381y Batchelor Apr. 15, 1941 2,266,920 Theile Dec. 23, 1941 2,414,792 Becker Jan. 28, 1947 2,414,793 Becker et al. Jan. 28, 1947 2,423,476 Billines et al. July 8, 1947 2,516,873 Havens et al Aug. 1, 1950 2,587,674 Aiken Mar. 4, 1952 2,633,521 Becker et al. Mar. 31, 1953 2,636,100 Anderson Apr. 21, 1953 2,742,550 Jenness Apr. 17, 1956 2,745,284 Fitzgerald May 15, 1956 2,839,645 Hester June 17, 1958 2,861,165 Aigrain et al Nov. 18, 1958 FOREIGN PATENTS 655,890 Germany Jan. 25, 1938 OTHER REFERENCES An Achromatic Doublet of Silicon and Germanium, by R. G. Treuting, Journal of the Optical Society of America, vol. 41, No. 7, July 1951, pp. 454-456.

Properties of Thermistor Infrared Detectors, by Eric M. Wormser, Journal of the Optical Society of America, vol. 43, No. l, January 1953, pp. 15-21.

Development and Operating Characteristics of Thermistor Bolometers by Becker et al., OSRD 5991, May 1946, chapters 2 and 3 relied on (9 pages). 

